nixos/phoebus-olog: use FerretDB intead of MongoDB

[minijackson]

To prevent having to compile MongoDB, which requires a lot of resources

[stephane-cea]

Nice workaround! Thanks

[minijackson]

Oh wait, if we change database backend, users will need to migrate their data from MongoDB to FerretDB, I'll need to document that. I'll also move that into the breaking changes.

[stephane-cea]

OK

I'll mark this pull request as a draft in the meantime.

[stephane-cea]

By the way, I just checked the known differences between MongoDB and FerretDB.

Something is drawing my attention:

document fields of double type must not contain Infinity, -Infinity, or NaN values.

update operations producing Infinity, -Infinity, or NaN are not supported.

Are we sure Olog isn't using any NaN? Because some PVs can hold NaN values...

[minijackson]

@stephane-cea Good question! I just checked, and it seems that Phoebus Olog only uses MongoDB to store attachments. Phoebus also doesn't currently seem to provide the option to send PV values in a log entry.

[stephane-cea]

In this case, I think it's safe to use FerretDB

Thanks for checking 👍